The formation of nitrogen oxides (NO.sub.x) in the case of most fuels, and of soot in the case of liquid fuels, are two problems always associated with combustion chambers. These problems tend to be accompanied by flame instability. The production of soot which accompanies the combustion of liquid fuels seem at first blush to be incompatible with the formation of NO.sub.x, since the former results from a deficiency of oxygen supplied by the air.
In order to deal with the problem of NO.sub.x, much work has been carried out on the recirculation of combustion gases. The object of this recirculation is to reduce the excess of the air necessary for combustion while increasing the mass flow rate. The result of this is to provide a better utilization of the oxygen without increasing the production of nitrogen oxides (NO.sub.x), which are a serious source of atmospheric pollution, and in general to provide the so-called "blue flame" combustion.
Of the solutions which have been proposed, one makes use of an air injection zone above or upstream of the fuel injection zone. This air injection zone is connected on the one hand to the fuel injection zone, and on the other hand to the combustion chamber. The air injected into the air injection zone causes a reduction in pressure, which draws the combustion gases from the chamber towards this zone and reinjects them, together with air, into the fuel injection zone. This solution has the disadvantage that a relatively long delay is necessary to obtain a satisfactory combustion. This delay is not tolerable, bearing in mind the frequent stopping and starting operations particularly in the case of a boiler.
Other solutions have been proposed to improve the combustion. One of these relates to the turbulent flow of the air feeding the burners and is generally known by the anglo-saxon expression "swirl". One chapter in the work of J. M. Beer and N. A. Chigier, "Combustion Aerodynamics", Applied Science Publishers Limited, London (1972) is devoted to this topic. This work shows in particular that the turbulent flow of the air around the fuel injection cone increases both the stability and luminosity of the flame, considerably alters the shape of the flame, which thus spreads very rapidly from the outlet of the fuel injection nozzle, and produces a high degree of mixing and turbulence. On the other hand, this improvement in the flame does not lead to sootless combustion, and is accompanied by a high level of noise due to the turbulence and the very high combustion rate. This noise level may even exceed the permissible limits.
Finally, there may also be mentioned an experiment which has been carried out on a gas burner in order to reduce the amount of NO.sub.x. In accordance with the experiment, four pipes lead tangentially into a tubular chamber which is concentric with a gas inlet pipe. Air and combustion gases are passed under pressure so as to form a "swirl" around the nozzle. However, these experiments have not gone beyond the laboratory stage and appear to have come up against stability problems. Furthermore, the laboratory prototype described does not appear to be easily applicable to industrial use.
The work which has been mentioned would seem to prove that it should be possible to perform combustion with a low rate of formation of NO.sub.x while approaching conditions of stoichiometric combustion, by virtue of the recirculation of the combustion gases on the one hand and the turbulent flow on the other hand. However, the solutions proposed do not in practice provide the combustion stability and cleanliness which ought theoretically to have been expected using recirculation techniques and turbulent flow.
The causes of this relative lack of success are doubtless many and varied. In the case of internal recirculation, the basic problem involves the delay in establishing a satisfactory combustion. The formation of a flow of combustion gas in the direction of the reduced pressure region in the air injection zone, upstream of the fuel injection nozzle, in fact takes a certain time to become established since the combustion gases in the combustion chamber are aspirated by the funnel on the one hand, and by the air injection zone on the other hand.
Another reason for this partial lack of success is manifested in a fairly high degree of flame instability, which is reflected in a fluctuating mode of combusion. Finally, the noise produced by yellow-flame combustion or by the turbulent type of flow may exceed the normally permissible level.
The instability of the flame is due to a large extent to a poor mixture of fuel and air and to insufficient dilution of the available oxygen. In order to obtain a satisfactory combustion, in particular one with a blue flame, the fuel if liquid must be atomized into sufficiently fine droplets. It is also necessary for the available oxygen to be sufficiently diluted in the gaseous mass formed from air and combustion gas. In fact, in order for the air excess to be reduced to a minimum and for the combustion thereby to approach stoichiometric conditions, it is necessary to ensure that each molecule of oxygen has a good probability of meeting a molecule of fuel. This is the reason why it is not sufficient to recirculate a certain amount of combustion gas: the oxygen must also be diluted in a homogeneous manner throughout the mass of gas.
In order to achieve this dilution it is desirable to use a gas whose partial pressure of oxygen is less than that of air, so as to exploit the greater part of the available oxygen, thereby leading to a reduction in the production of NO.sub.x. However, the reinjection methods so far used have not ensured a sufficiently good dilution of the oxygen. Consequently, since the mass of gas does not have a constant partial pressure of oxygen in the reaction zone with respect to time, the combustion may not be uniform.
The stability of the flame cannot be guaranteed even by combining turublent flow and recirculation, doubtless for the same reason, namely an incomplete dilution of the available oxygen in the mass of gas.